The present invention relates to rotary printing presses, and in particular the present invention relates to a rotary offset printing press having at least one plate cylinder and at least one blanket cylinder.
In a rotary offset printing press the plate cylinder is disposed in rolling engagement with the blanket cylinder which in turn contacts the web or sheet to be printed. The web or sheet is backed by either an impression cylinder or, in the case of a perfecting press, another blanket cylinder.
There are various known general constructions of offset printing presses, and each of these enables the cylinders to be separated or thrown off from each other. This has been done by mounting one of the cylinders in eccentric bushings called throw off eccentrics. In one common general construction the blanket cylinder is mounted in throw off eccentrics, and throw off is achieved by movement of the blanket cylinder relative to the plate cylinder and the frame. In another general construction the plate cylinder is mounted in the throw off eccentrics so that throw off is achieved by movement of the plate cylinder relative to the press frame and blanket cylinder while the blanket cylinder remains stationary. This arrangement has been used in perfecting presses in which the plate cylinders and blanket cylinders rotate about axes which lie in a common plane.
In addition to mounting the plate and blanket cylinders so that they are relatively movable for purposes of throw off, it has also been a common practice to mount the plate cylinder so that it may be skewed relative to the blanket cylinder. To this end, the plate cylinder has been mounted at each end in self-aligning bearings, and at one end the self-aligning bearing is mounted in an eccentric called the skew eccentric. When the skew eccentric is rotated, the end of the plate cylinder which it supports moves in an arcuate path and the other end of the plate cylinder pivots about the self-aligning bearing supporting that end. This varies or skews the line of contact between the plate and blanket cylinders. Skew adjustment provides proper registration in the event that the printing plate is improperly aligned when attached to the plate cylinder, as is well known in the art.
Ink is applied to the plate cylinder by a rubber form roll which runs in engagement with the plate cylinder. The amount of ink which adheres to the plate depends in part on the contact pressure between the form roll and the printing plate. Rotary presses have means for adjusting the center-to-center distance between the form roll and the plate cylinder to vary the contact pressure. Because the form roll contact pressure adjustment is sensitive, skew movement of the plate cylinder changes the form roll contact pressure adjustment. To overcome this difficulty, numerous devices have been designed to enable the form roll to plate cylinder center-to-center distance to be maintained during skew movement of the plate cylinder. Examples of such devices may be found in U.S. Pat. Nos. 3,538,849 and 4,214,527.
In preparing a rotary printing press for operation it is desirable to be able to throw the form roll into and out of rolling engagement with the plate cylinder. Once the center to center distance between the form roll and plate cylinder has been adjusted, it is desirable to be able to throw off the form roll and to return it to engagement with the plate cylinder to the previously selected distance. Devices to accomplish this are also disclosed in the above-mentioned U.S. patents.
To the applicant's knowledge there are no printing presses in which the plate cylinder is moved for throw off from the blanket cylinder and may be skewed relative to the blanket cylinder and in which the form roll adjustment is automatically maintained during throw off and skewing of the plate cylinder.
The present invention comprises a rotary offset printing press in which a plate cylinder is mounted in eccentrics for the purpose of throwing off the plate cylinder from a blanket cylinder. In addition, one end of the plate cylinder is mounted in an eccentric for making skew adjustments of the plate cylinder relative to the blanket cylinder. A form roll is mounted in rolling contact with the plate cylinder, and the center-to-center distance between the form roll and the plate cylinder may be adjusted to achieve a desired contact pressure and the adjustment is maintained during both throw off and skewing of the plate cylinder.
A cam is provided at each end of the plate cylinder. The cams are rotatable about the axis of the plate cylinder and they are mounted on the bearings which support the plate cylinder. Therefore, the cams move whenever the plate cylinder is moved, either for throw off from the blanket cylinder or for skew adjustment. Moreover, the cams are rotatable independently of both the skew and plate throw off eccentrics for throwing the form roll off the plate cylinder. Therefore, throwing off the plate cylinder does not effect the form roll to plate cylinder center-to-center adjustment, and the form roll can be thrown on or off the plate cylinder whether or not the plate cylinder is thrown on or off of the blanket cylinder.